1. Open the terminal by
Navigate to Applications -> Utilities -> Terminal
2. Type command below and hit enter
Mac OSX 10.7 & 10.8 (Lions) : sudo killall -HUP mDNSResponder
Mac OSX 10.5 & 10.6 (Leopards) : dscacheutil -flushcache
Mac OSX 10.4 (Tiger) : lookupd -flushcache